Social voting norms persistently impel citizens to the polls. To date, most research in this field has focused on norms coming from the community at large rather than voters’ particular social groups. But pressure to conform to in-group norms may have an even stronger effect; inquiry across disciplines repeatedly demonstrates that group identity can be an important moderator in the relationship between norms and behavior. We apply this lesson to political behavior, testing the effect of partisan social pressure on turnout. We report the results of a randomized field experiment conducted during the 2012 Iowa primary election, comparing the mobilization effects of partisan and nonpartisan direct mail messages. We test the interaction between social pressure and the partisan nature of the message and find that partisan direct mail messages alone do not effectively mobilize voters. When partisan and social pressure elements are combined, turnout increases, but no more so than when communitarian and social pressure elements are combined. We conclude that simply referencing a voter’s party does not seem to render mobilization messages more effective.